The variability of precipitation generally increases from the northwest to the southeast portion of the state and the number of daily rainfall totals exceeding one-half inch per day varies from about 15 days in the northwest to 25 days in the southeast. Nearly three-fourths of the annual precipitation falls during the April through-September growing season, with measurable rainfall (>= 0.01 inch) occuring on about 100 days per year. The table below shows the variation in the monthly rainfall totals across the state.
| Jan | Feb | Mar | Apr | May | Jun | Jul | Aug | Sep | Oct | Nov | Dec | Annual | |
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Burlington | 1.24 | 1.26 | 2.79 | 3.76 | 4.85 | 4.47 | 4.25 | 4.27 | 3.52 | 3.12 | 2.50 | 2.09 | 38.48 |
| Des Moines | 1.00 | 1.28 | 2.30 | 3.86 | 4.74 | 4.94 | 4.47 | 4.13 | 3.05 | 2.64 | 2.19 | 1.42 | 36.02 |
| Shenandoah | 0.90 | 1.15 | 2.30 | 3.24 | 5.76 | 5.53 | 4.63 | 3.74 | 3.22 | 2.55 | 2.04 | 1.28 | 36.34 |
| Sioux City | 0.62 | 0.67 | 2.00 | 2.95 | 3.74 | 3.89 | 3.44 | 3.23 | 2.96 | 2.13 | 1.31 | 0.80 | 27.74 |
| Waterloo | 0.83 | 0.99 | 2.06 | 3.71 | 4.53 | 4.98 | 4.91 | 4.27 | 2.63 | 2.48 | 2.01 | 1.20 | 34.60 |